It is well known in the art that pipeline girth (circumferential) welds are often inspected with radiography using a conventional x-ray crawler in conjunction with either x-ray film or real-time radiographic (RTR) detectors. These crawlers are used when access can be easily made to an open end of a pipeline section which is being welded to another pipeline section. The x-ray crawler comprises an x-ray source on a crawler or buggy which can be driven into the open end of the pipeline and which will crawl along the pipeline to the area of the circumferential weld.
The x-ray source is panoramic and mounted to be substantially central within the pipe and emits x-rays around a 360 degree arc around the weld surface. This type of x-ray source is generally used with x-ray film and is suitable for most pipe diameters.
Use of x-ray film requires time consuming and environmentally unfriendly chemical processing, washing and drying prior to the production of an image which can be viewed and stored.
RTR detectors may be used with a pipe center mounted panoramic x-ray source. However, their applications are typically limited to pipe diameters of 24 inches or less, as inspection times on larger diameters increase rapidly. This arises because as the pipe diameter increases, the intensity of x-ray flux at the weld falls off in accordance with the inverse square law as distance from the x-ray source increases. The result of this is that on medium to large diameter pipes, the x-ray flux is of such a reduced level that inspection with RTR detectors is unacceptably slow and therefore not commercially viable.